SMTP Test
Connect to a mail server, check its TLS, try the login and send yourself a test email - one step at a time, with the server's own replies, so you can see exactly where it breaks.
Which port and security to use
| Port | Security | Use it for |
|---|---|---|
| 587 | STARTTLS | Apps and mail clients sending mail. The connection starts in plain text and upgrades to TLS before the login. The default almost everywhere. |
| 465 | SSL/TLS on connect | The same job, encrypted from the first byte. Many providers offer both 465 and 587. |
| 25 | STARTTLS if offered | Server-to-server delivery. Most home connections and cloud hosts block it outbound - including the server this tester runs on - so don't use it for an app. |
| 2525 | STARTTLS | An unofficial fallback some services (SendGrid, Mailgun, Brevo) offer for networks that block 587. |
Auto picks TLS on connect for port 465 and STARTTLS (when the server offers it) for everything else - the right guess nearly always. The classic mistake is the reverse: SSL/TLS on port 587, or STARTTLS on 465. The connection then hangs or fails the handshake, and this tester says which way round it should be.
SMTP settings for popular providers
| Provider | Server | Port | Log in with |
|---|---|---|---|
| Gmail | smtp.gmail.com | 587 STARTTLS or 465 | Your full address and an app password (needs 2-Step Verification) |
| Microsoft 365 | smtp.office365.com | 587 STARTTLS | Your address and password - if SMTP AUTH is enabled for the mailbox |
| Outlook.com | smtp-mail.outlook.com | 587 STARTTLS | Your address; Microsoft is moving consumer accounts to OAuth-only logins |
| Yahoo Mail | smtp.mail.yahoo.com | 587 STARTTLS or 465 | Your address and an app password |
| iCloud Mail | smtp.mail.me.com | 587 STARTTLS | Your iCloud address and an app-specific password |
| Zoho Mail | smtp.zoho.com | 587 STARTTLS or 465 | Your address (EU accounts: smtp.zoho.eu) |
| Fastmail | smtp.fastmail.com | 465 or 587 STARTTLS | Your address and an app password |
| SendGrid | smtp.sendgrid.net | 587 STARTTLS | User apikey, password = an API key |
| Mailgun | smtp.mailgun.org | 587 STARTTLS | The domain's SMTP credentials (EU: smtp.eu.mailgun.org) |
| Amazon SES | email-smtp.<region>.amazonaws.com | 587 STARTTLS | SES SMTP credentials for that region - not your IAM keys |
| Brevo | smtp-relay.brevo.com | 587 STARTTLS | Your SMTP login and SMTP key from the Brevo dashboard |
Providers change these now and then; when this table and the provider disagree, the provider wins - and the tester above will show you which one is right.
What the errors mean
535 5.7.8 Username and Password not accepted- Gmail's wrong-credentials reply. With the right password it almost always means an app password is needed.534 5.7.9 Application-specific password required- Gmail again, saying it directly.535 5.7.3 Authentication unsuccessfulor5.7.139- Microsoft. Wrong credentials, SMTP AUTH turned off for the mailbox, or password logins disabled for the tenant.530 5.7.0 Must issue a STARTTLS command first- the server wants encryption before the login. Set Security to STARTTLS or Auto.550or554 ... relay/sender not allowed- logged in, but not allowed to send from that From address. Use the account's own address or a verified alias.421- the server is busy or limiting connections. Wait a minute.- Timeout on connect - a firewall dropping the port. On port 25 that's normal; try 587.
Common questions
Does it send an email?
Only if you tick the box, and only after the login succeeds - the tester never relays mail without a login. It sends one message with a fixed text to one address, and allows three of those an hour.
Where does the test run?
From our tools server in Amsterdam, Netherlands - a browser can't open a raw SMTP connection. It can only reach public servers; a mail server on your LAN or on localhost isn't reachable from here.
Which settings does UpWatchr's email alert need?
The same host, port, user and password you tested here, with SSL on for STARTTLS on port 587 (or off for an unencrypted server). It doesn't connect with TLS-on-connect on port 465, so test 587. UpWatchr then emails you whenever a site, API or server you watch stops answering - or alerts you by popup, Telegram or webhook instead.